Geology of California

Lesson 17: Peninsular Ranges and Historical Overview

Our final lesson covers two relatively brief chapters. Chapter 17 introduces the Peninsular Ranges province which lies south of the Transverse Ranges. The Peninsular Ranges are dominated by a batholith that was formed as a result of Mesozoic subduction and is strikingly similar in age and host rock compositions to the Sierra Nevada batholith. One group of rocks in the Penninsular Ranges batholith that really does not have any counterpart in the Sierra Nevada, however, are the gem-bearing pegmatites of San Diego County. These pegmatites, which crystallized from the final, water-rich fractions of the batholith's magmas, are rich in rare "incompatible" elements and contain open cavities that have yielded millions of dollars of gem-quality tourmaline, beryl, and topaz (a reconstructed pegmatite "pocket" from the LA Natural History Museum is shown at right). West of the batholith sedimentary strata preserve a large submarine fan with rocks similar to those of the Great Valley sequence, and the San Onofre breccia contains clasts of rocks from deep in an underlying subduction zone that were brought to the surface as Cenozoic extension fragmented southern California.

Next, chapter 18 reviews many of the major events in California's geologic history—from late Proterozoic rifting in eastern California to the Cenozoic development of the San Andreas fault—that we've studied during previous lessons. Rather than linking these events to specific provinces, however, this chapter gives you an opportunity for you to see how they all "fit together"so that you can gain a broad overview of California's development. It will also give you a little time to think holisitically about how what you've learned in specific chapters fits together as you begin to prepare for our final. Finally, this week's exercise introduces you to the the techniques geologists use to assess flood hazards, which are an annual threat when winter rains come to the mountains of southern California.

As you read about the Penninsular Ranges and review California's geologic history be sure to take careful notes on the topics covered by the learning objectives below. As during previous weeks, you'll find that writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and spot any gaps in your knowledge. Don't hesitate to post any questions you have to the Discussion Board so that your classmates or I can help you figure them out. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refer back to it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.

Weekly Learning Objectives

Upon successful completion of this week's lesson, a student is expected to be able to:

Describe how the eastern and western parts of the Penninsular Ranges batholith (PRB) differ in age and composition, and briefly explain how these differences are related to the batholith's
"assembly" on different types of crustal basement (accreted oceanic lithosphere covered by turbidites in the west versus ancient continental lithosphere covered by a carbonate platform in the east).

Describe how the grain size and types of minerals present
would enable you to distinguish a pegmatite from a typical plutonic rock
in the PRB, and explain why these differences developed.

Describe how you could distinguish sedimentary rocks of the Cabrillo Formation from the San Onofre breccia, and briefly explain what each unit tells us about
the ancient geography of the San Diego region when it was deposited.

Cite two features of the San Diego region which indicate it is tectonically
active today, as it has been through much of late Cenozoic time.

Order a list of the major events in California's geologic history in their correct sequence from oldest to youngest.

Given a paleogeographic
map of California, tell what approximately which period of the state's geologic history it depicts and cite observations to justify his or her conclusion.

Reading and Browsing Assignment

Read Chapters 17 and 18, focusing on the topics outlined in the learning objectives above.

To learn a little more about pegmatites check out the Pegmatite Zone (page colors are a little gaudy, but links at the bottom will take you to pictures of some of the key mines in San Diego County and photos of the minerals they've yielded). Also, check out this virtual tour of the Stewart Mine from MMM Gems.

Finally, for a brief text-only introduction check out the Geology of San Diego County by Thomas Demere at the San Diego Natural History Museum.

In a typical year Southern California's Penninsular and Transverse Ranges suffer through fall wildfires that are fanned by the Santa Ana winds. As devastating as these fires can be, however, the fact that they denude the landscape of vegetation means that they set the stage for landslides and flooding that accompany the winter rains. With California's climate warming and drying, the threat of wildfires and subsequent landslides and flooding is only likely to increase in the future. To learn how geologists and urban planners assess flood hazards, please load up your Hazard City CD and work through version 1 of the Flood Insurance Rate Maps exercise. I suggest printing the worksheet provided on the report page and just filling it in as you go. When you're done, go to the "Assignments, Tests, and Surveys" tool in Etudes and report your findings by answering the questions in Exercise 17. Note that the exercise has nine questions and that I have added a tenth related to the base flood elevation near site 2.

After you feel you have met the learning outcomes outlined above please complete Quiz 17 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about California's Penninsular Ranges and its geologic history, and each is worth one point. If you can answer all of them correctly it means that you know your way around the geology of the Peninsular Ranges pretty well, have a handle on the "big events" in California's geologic history, and are ready to use what you've learned this semester to complete our final next week (see below). Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.

Our last assignment this semester will be the final exam. It's worth 45 points and will have 60 questions (that's 0.75 points per question). It will be written in two parts, each with 30 questions, and you will have 50 minutes to take each part. The two parts do not have to be taken consecutively. For example, you can take one part on one day and the other part on the next. You do have to be finished with both parts by 5:00 PM on Thursday, 19-May, however, so that I can submit final grades by the College's deadline. The questions on the exam will be based on the learning objectives we've had for all of our weekly lessons and will be similar (although maybe not identical) to questions you've seen before. A good way to study for the final will be to go back and review your old quizzes and make sure that you understand anything you missed the first time. I'll provide more details about the final in an announcement I'll post on Sunday, 15-May, but for now it probably would be good to take some time and start your review. If you have any questions just post to the discussion board or send me a private message. As usual, you'll want be sure to' use Firefox when you take the final.